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speciation; population genetics; bird colouration; pigmentation genes; recombination rate; linkage disequilibrium
Abstract:
Colouration patterns have an important role in adaptation and speciation. The European crow system, in which all-black carrion
crows and grey-coated hooded crows meet in a narrow hybrid zone, is a prominent example. The marked phenotypic difference
is maintained by assortative mating in the absence of neutral genetic divergence, suggesting the presence of few pigmentation
genes of major effect. We made use of the rich phenotypic and genetic resources in mammals and identified a comprehensive
panel of 95 candidate pigmentation genes for birds. Based on functional annotation, we chose a subset of the most promising
37 candidates, for which we developed a marker system that demonstrably works across the avian phylogeny. In total, we
sequenced 107 amplicons (B3 loci per gene, totalling 60kb) in population samples of crows (n¼23 for each taxon). Tajima’s D,
Fu’s FS, DHEW and HKA (Hudson–Kreitman–Aguade) statistics revealed several amplicons that deviated from neutrality; however,
none of these showed significantly elevated differentiation between the two taxa. Hence, colour divergence in this system may be
mediated by uncharacterized pigmentation genes or regulatory regions outside genes. Alternatively, the observed high population
recombination rate (4NerB0.03), with overall linkage disequilibrium dropping rapidly within the order of few 100bp, may
compromise the power to detect causal loci with nearby markers. Our results add to the debate as to the utility of candidate gene
approaches in relation to genomic features and the genetic architecture of the phenotypic trait in question.
